The present application relates to a thin film transistor (TFT) having an oxide semiconductor layer as a channel (active layer), a method of manufacturing the same, and a display device using the thin film transistor.
Oxide semiconductor such as zinc oxide, indium gallium zinc oxide (InGaAnO), or the like displays excellent property as an active layer of a semiconductor device. In recent years, the oxide semiconductor is being developed with aim for applications to a TFT, a light emitting device, a transparent conductive film, and the like. For example, a TFT using oxide semiconductor has electron mobility higher than that of a TFT using amorphous silicon (a-Si: H) which is used in an existing liquid crystal display device, and has excellent electric property. There is also an advantage that high mobility is expected even at low temperature around the room temperature.
On the other hand, it is known that, heat resistance of the oxide semiconductor is insufficient, oxygen, zinc, and the like desorbs by heat treatment in the TFT manufacturing process, and a lattice defect occurs. The lattice defect electrically causes shallow impurity level and lower resistance in an oxide semiconductor layer. Consequently, the operation becomes an operation of a normally-on type, that is, depletion-type in which drain current flows without applying gate voltage. The defect level becomes higher, the threshold voltage decreases, and leak current increases. There is consequently a disadvantage such that the characteristic fluctuation of a TFT using oxide semiconductor is large (J. Non-Crystalline Solids 354 (2008) 2826).
To address the disadvantage, a technique is being proposed that, for example, a gate insulating film which is in contact with a channel layer made of oxide semiconductor is made of amorphous aluminum oxide (Al2O3) and used as a sealing film that reduces the defect level in the interface (see, for example, Japanese Patent No. 3,913,756).